Magnetic garnets for microwave frequencies



March 17, 1964 J, E. zNl-:IMER ETAL 3,125,534

MAGNETIC GARNETS FOR MICROWAVE FREQUENCIES Filed June 8, 1961 2 sheets-sheet 1 /3 AMX/Adam4 oss i 26 A 77E/V04 T/oN L oss (aga/556) @@mw-QM 4 TTMs (Gauss) March 17, 1964 Filed June 8, 1961 J. E. zNElMER ETAL 3,125,534

MAGNETIC GARNE'lS FOR MICROWAVE FREQUENCIES 2 Sheets-Sheet 2 zooo | eoo soo 5o |00 |50 20o 25o 50o TEMPERATURE (c) MAGNETIZATION vs TEMPERATURE /fvvENroRs JOEL E. ZNE/ME? MARSHALL H. SIRVE'TZ ATTORNEY United States Patent C 3,125,534 MAGNETIC GARNETS FOR MICRGWAVE FREQUENCIES Joel E. Zneimer, Verona, NJ., and Marshall H. Sirvetz,

Boston, Mass., assignors to Raytheon Company, Lexington, Mass., a corporation of Delaware Filed .lune 8, 1961, Ser. No. 120,118 8 Claims. (Cl. 252-625) This application is a continuation-in-part of application, Serial No. 714,158, filed February 10, 1958, by Joel E. Zneimer et al., now abandoned.

This invention relates generally to ferromagnetic materials and more particularly to those of the garnet crystal structure, hereafter caller garnets.

In the past, ferrites have been used in microwave equipment, for instance, to provide predetermined phase shifts (directional phase Shifters) phase reversals (gyrators) and high attenuation ratios between forward and backward waves (isolators). These materials, composed of oxygen, iron, and usually other metallic elements of the iron group, have the spinel crystal structure and have been found to be particularly useful in these applications because of their high resistivities and, consequently, small insertion losses. Diiiiculties have been encountered', however, with spinel ferrites because of their poor operation in the lower frequency portions of the microwave spectrum. Spinel ferrites presently available are useful for frequencies only down to approximately 60() megacycles.

This invention covers ferromagnetic materials composed of oxygen, iron, and any one or combination of the ions that form the garnet crystal structure. Among these ions are yttrium and many of the rare earth ions (for example, lutecium (Lu), gadolinium (Gd), Samarium (Sm) These materials provide a means for operating over a very large range of frequencies especially in the lower frequency portions of the microwave region. This is made possble by allowing independently controlled variations of the saturation magnetization and the ferromagnetic resonance line width. These independent variables give rise to a large range of variations in garnet characteristics in accordance with particular applications that may be required. The possible combinations available and their characteristics may be best explained with the help of the drawing in which:

FIG. l shows a graph representing a typical ferromagnetic resonance curve;

FIG. 2 shows a graph depicting variations in ferromagnetic resonance line width and saturation magnetization for a particular embodiment of the invention;

FIG. 3 shows a graph depicting variations in ferromagnetic resonance line width and saturation magnetization for another particular embodiment of the invention;

FIG. 4 shows a cross-sectional view of a microwave device that represents a specific embodiment of the invention; and

FIG. 5 shows a graph depicting variations in saturation magnetization as a function of temperature for different yttrium-gadolinium garnet mixtures.

In typical resonance absorption isolators, for example, where ferrite or garnet materials are placed in wave guides and a transverse magnetic field of resonance value, H0, is applied, a wave propagated in one direction is strongly attenuated while a wave propagated in the opposite direction is relatively slightly affected. FIG. 4 shows a typical isolator utilizing a garnet material for use in the microwave region.

In the ligure, the isolator is a rectangular wave guide structure 21 which is shown in a cutaway view looking down the opening 22 of the wave guide in the direction of propagation. The wave guide has rectangular end MZSE Patented Mar. 17, 1%64 ICC pieces 23 and 2.4 which are used to connect the isolator to other wave guide sections of a microwave wave guide system. Thin slabs 2S of garnet material are mounted symmetrically on the upper and lower interior surfaces of the wave guide, as shown. Permanent magnets 26 are mounted on the upper and lower exterior surfaces of the wave guide with their pole faces in positions directly over the garnet slabs.

An electromagnetic wave entering the wave guide structure at the left opening 22 will be transmitted with negligible loss of energy so that essentially all of the wave is propagated through the isolator from left to right. A wave entering at the right, however, will be transmitted with a very large loss in energy so that essentially none of the wave is propagated through the isolator from right to left. Such a wave guide structure as herein described represents only one embodiment of the invention and many more will occur to those skilled in the art.

A typical resonance curve 14 is shown in FIG. l in which attenuation loss (in decibels) is plotted as a function of applied magnetic field in oersteds. FIG. 1 shows that AH is dened as the width 16 of the resonance curve (in oersteds) measured between the points 11 and 12 at which the value of the resonance curve is one-half its maximum value 13, designated in the figure as maximum loss. H0 is defined as the value of magnetic eld strength at which maximum attenuation occurs, as shown at point 2th The resonance curve 14 is measured at a particular value of applied frequency, f, according to conventional methods well known in the art. It is well known that if the line-width AH is small, the ratio of attenuation loss is large, so that for good isolation it is desirable to minimize AH. As the frequency is decreased, the attenuation ratio decreases. For Spinel ferrites the minimum obtainable line width is about oersteds, and this has limited these ferrite devices to frequencies above about 600 me.

Another important factor to be considered in microwave applications of ferrite or garnet devices is the band width of operation. lt is well known that the band width of an isolator is proportional to AH according to the expression:

Af=fy times AH in which Af is the band Width measured in megacycles, AH is the line width measured in oersted, and the proportionally constant 'y (known as the gyromagnetic ratio) is dependent upon the material used for the isolator. For garnet devices the gyromagnetic ratio y is approximately equal to 2.8. Thus, a large band width requiring large AH is not compatible with a large attenuation ratio requiring small AH. Because a compromise must be reached, it is highly desirable that a variable AH be available for the particular application of the device.

Another important factor to be considered in microwave applications is the saturation magnetization, MS. At high microwave frequencies a high saturation magnetization is ordinarily desirable. However, if it is necessary to operate in the low frequency region, a high saturation magnetization may cause undesirable losses at low applied magnetic iield strengths. For some materials the resonance curve may have appreciable losses at the low end of the resonance curve, as shown by dashed line `l5 in FIG. l. For lower saturation magnetizations, this loss at the low end becomes negligible. It is known that the minimum operable frequencies depend on M, and can be indicated by the expression:

.icmlnz'll tifnes (47VMS) in which imm is measured in megacycles, (4m-MS) is measured in gauss and the proportionality constant ly is dependent upon the material used. For garnet devices y is approximately equal to 2.8. Thus, it is desirable to E3 have a means of varying the saturation magnetization Ms.

This invention discloses that it is possible to provide independent variation in the magnetization and line width by varying the composition of garnets in a predetermined manner. The class of materials that belong to the garnet crystal structure group have the formula M3Fe5012, which may be alternatively written as 3M2O35Fe203- In this formula M can be, `for example, any predetermined combination of ytrrium, lutecium, gadolinium and Samarium. The latter three elements are members of the group known as the rare earth metals having atomic numbers 8 through 71. Samarium, gadolinium and lutecium have atomic numbers 62, 64 and 71, respectively. Yttrium, while not conventionally classified as a rare earth metal, is structurally and chemically similar to the rare earths and is often found in nature with the rare earths. The magnetic garnets are all found to have high resistivity and Curie temperatures of approximately 300 C. These properties, similar to those of the spinel lferrites, allow the use of this garnet structure group in microwave ferrite applications.

Resonance measurements `of the garnets formed with same of these materials indicate that yttrium garnet has a relatively `small line Width of the order of 70 oersteds measured at both 10,000 megacycles and 1300 megacycles, while samarium garnet, on the other hand, has a large line width of the order of 20003000 oersteds depending on the temperatures at which it was fired in its preparation.

A mixed crystal can be formed from a combination of the above materials to provide a garnet that has a line width AH variable over a wide range. This mixed crystal has the formula (Y1 Sm)3Fe5O12. As a is varied over the range from 0 to 1, the composition is varied from the garnet of yttrium 01:10), to a garnet that is a mixture of samarium with yttrium (0 a 1), and finally to the sarnarium garnet (a=1). Solid curve 16 of FIG. 2 shows the variation of AH as a function of a for such an yttrium-samarium garnet. The measurements were made at 10,000 megacycles 4at room temperature and the variation in AH went from 85 `oersteds for an yttrium garnet to 3200 oersteds for a samarium garnet. In this type of garnet, while AH is Varied, the saturation magnetization Ms remains essentially constant, as shown by the dashed line curve 17. In FIG. 2, the values shown on the ordinate designate either AH (as measured in oersteds) `or 11i-MS (as measured in gauss).

A first class of mixed garnets having the general characteristics, shown by the yttrium-samarium garnet described above, may be formulated more generally in the following manner. A rst class of garnets A3Fe5012 may be formed in which A represents an element or combination of elements capable of forming a garnet having a small line width AH. A second class of garnets M3Fe5012 may be formed in which M represents an element or combination of elements capable of forming a garnet having a large line width AH. A mixed garnet may be prepared having the general formula (AlgaMn) 3Fe5O12 in which a variation in a provides a variation in line width over a wide range of values, such as has been shown, for example, in FIG. 2 for the yttrium-Samarium mixture. A and M are ordinarily considered to be elements in the plus three valency state, as A+3 and M+3. It is theoretically possible that an effective plus three valency state can be achieved by the use of a mixture of ions that form a garnet but which in themselves may not necessarily be in the plus three state. An example of this latter structure would be a mixture of one-half A1+2 and one-half A2M, or, similarly, one-half M1+2 and one-half M2M. Such latter mixtures theoretical-ly may be formed wit-hout limiting the scope and applicability of the general formula (A1-aMa)aFe5O12- Because lutecium (Lu) as well as yttrium forms a narrow line Width garnet, the symbol A in the general formula may be yttrium, lutecium or a combination of both. With the exception of yttrium and lutecium, the garnets in general are known to have large line widths. Hence, M in the general formula may represent any of the elements forming a garnet with large AH, or a combination of them. Gadolinium garnet, however, which has a large line width in the vicinity `of room temperature, is an exception to this rule and cannot be used in place of M for reasons that will be explained below.

A second class of mixed crystals can be formed from a combination of some of the above-'mentioned materials to provide a garnet structure that has a saturation magnetization Ms variable over a wide range. This second class of garnets has the formula (A1 ,Gd)3'Fe5O12. In this formula A again represents yttrium or lutecium or a combination of both and Gd, of course, represents gadolinium. As is varied over limits from 0 to l, the composition is varied from a garnet of yttrium, lutecium or combination of both (,B=0), to a garnet that is a mixture of gadolinium (Gd) with yttrium, lutecium or combination of both (0 1), and finally to a gadolinium garnet 1). Dashed curve 18 of FIG. 3 measured at room temperature shows the variation of 41rMs as a function of for an yttrium-gadolinium garnet having the formula The variation in 41rMS is from 1965 `gauss for an yttrium garnet to 90 gauss for a gadolinium garnet. In this type of garnet, as Ms is varied, the line width AH remains fairly small until percent or more of gadolinium is present, as shown in the solid curve 19 of FIG. 3. In FIG. 3, the values shown on the ordinate designate either 41rMs (as measured in gauss) or AH (as measured in oersteds).

The behavior of the yttrium-gadolinium garnet can be attributed to the 'fact that the gadolinium garnet has a magnetic compensation temperature point in the vicinity of room temperature. At that compensation point (sometimes called a second Curie point) the magnetization is very small. As the temperature is changed in either direction about this point the magnetization increases. The compensation point is known to contribute in large part to the fact that the gadolinium garnet has a large line width in the vicinity of room temperature. However, as the temperature changes about that point, the line width decreases rapidly, showing that the broad resonance at room temperature is due to the magnetic cornpensation and not to a characteristic property of gadolinium itself.

It has been found in this invention that when moderate amounts of gadolinium are added to yttrium to form a mixed garnet the compensation point exists 'at some vicinity other than room temperature and the line width of the mixed garnet is therefore small at room temperature. As the amount of gadolinium is increased, the compensation point approaches the vicinity of room temperature and the line width increases appreciably, as shown by the solid l-ine 19 of FIG. 3.

As the gadolinium content is increased in the vicinity of room temperature, the saturation magnetization MS decreases until =1 at which point the saturation magnetization is very small and the line width very large, as shown by curves I3 and I9 of FIG. 3. Because of this property of gadolinium it is thus possible to provide a mixed garnet of the form (A1 Gd)3Fe5O12 which has a variable Ms and a fairly constant line width AH when A is yttrium, lutecium or a combination of both.

A major advantage of the yttrium-gadolinium garnet mixture lies in the temperature stability of its saturation magnetization characteristics. For example, FIG. 5 shows a graph of the saturation magnetization for a number of different yttrium-gadolinium garnet mixtures as a function of temperature. Examination of this graph shows that at approximately room temperature (in the vicinity of about 25 C.) the magnetization decreases as the ratio of gadolinium content to yttrium content increases. This characteristic is perhaps more clearly shown in FIG. 3 where the magnetization shown by dashed line 18 is plotted as a function of (which as explained in previous paragraphs is a measure of the ratio of gadolinium to yttrium content) at room temperature.

As can be seen in the graph of FIG. 5, an increase in gadolinium content improves the temperature stability of the saturation magnetization. For example, upper curve 30 in FIG. 5 shows the change in the value of saturation magnetization of a substantially pure yttrium garnet as the temperature is increased from room temperature to approximately 300 C. Over that temperature range the magnetization varies over extremely wide limits from almost 2000 gauss to less than 50 gauss. Over a smaller range of temperatures up to 200 C. the value of the saturation magnetization varies over a range of approximately 1000 gauss from about 2000 gauss down to about 1000 gauss.

As the gadolinium content is increased to form a mixed garnet having the formula (Y,9Gd,1) 3Fe5012 (where lB=0.l), curve 31 shows that the yttrium-gadolinium garnet mixture has an improved temperature stability such that the saturation magnetization now Varies only over a range of less than 800 gauss from approximately less than 1700 gauss to approximately 900 gauss in the temperature range from about room temperature to 200 C.

Curve 32 shows the variation of magnetization with temperature of a mixed garnet having the formula (Y 75Gd.25)3F5012 (Where If equal parts of gadolinium and yttrium are utilized in a mixed garnet (wherein 13:05), the stability is improved further, as shown by curve 33, such that the magnetization varies only over limits of approximately 200 gauss (from about 900 gauss to about 700 gauss) up to temperatures of approximately 200 C. Even better ternperature stability is obtained for a mixed garnet wherein [3:06, as shown by curve 34, or a garnet wherein =0.75, as shown by curve 35, such that over a temperature range from room temperature to 200 C. the magnetization varies only about 100 gauss or less.

In any case, over at least a temperature range from room temperature to 200 C., most of the yttrium-gadolinium garnet mixtures are relatively stable with respect to temperature insofar as the change in their magnetization is concerned in comparison to substantially pure yttrium iron garnets. In all cases, the line width of the yttrium-gadolinium mixed garnet remains relatively small until reaches values above about 0.7, as shown by curve 19 of FIG. 3.

Therefore, in those applications wherein the environment in which the garnet is to be used varies over relatively wide temperature ranges, it is preferable to utilize the yttrium-gadolinium garnet mixtures as described above. An example of the calculations required to formulate an yttrium-gadolinium mixture in terms of its weight percentages can be shown in the following paragraphs.

The yttrium-gadolinium garnet mixture can be defined in accordance with the following formula:

(Y1pGdp) 31265012 (l) This formula can be rewritten in an equivalent form as follows:

(3 *X)Y203.XGC1203.5FC203 wherein x is equal to 3)? and the coeicients of the oxides epresent the required number of mols of each.

As one example, let it be assumed that equal mols of yttrium oxide and gadolinium oxide are required, in which case in Formula l will be equal to 0.5 and x in Formula 2 will be equal to 1.5. Thus Formula 2 becomes:

1.5Y203.1.5Gd203.5Fe203 Knowing the molecular weights of Y2O3, Gd203 and Fe203, a calculation can be made of the percentages by weight of Y2O3, Gd203 and Fe203 required to provide a composition as defined in Formula 3 in accordance with the following table.

Thus, for a 1000 gram batch of material it is necessary to mix 201.52 grams of Y2O3, 323.48 grams of Gd203 and 475 grams of Fe203.

Other yttrium-gadolinium garnet mixtures can be determined in a manner similar to that shown in the above Table I. Table II, which follows, shows the weight percentage requirements for various yttrium-garnet compositions over a desirable range of values.

TABLE Il Percentage by Weight IB YzOa GdzOa F0203 It has been found that yttrium-gadolinium garnets, wherein the yttrium oxide may vary in percentage by weight from approximately l0 percent to 40 percent, the gadolinium oxide may vary in percentage by weight from approximately 5 percent to 45 percent and the ferric oxide may vary in percentage by weight from approximately 45 percent to 60 percent, provide highly advantageous ternperature stabilized materials for use in microwave devices.

The `selection of a particular yttrium-gadolinium garnet composition depends in large measure on the particular application in which the composition is to be used. In those applications wherein improved temperature stability is desired and a minimum line width value is required, it is desirable to select an yttrium-gadolinium garnet in which the mol percentage of Agadolinium is about 10 percent to 20 percent and that of yttrium is about 80 percent to percent equal to about 0.1 to 0.2). Such a garnet provides a narrow line width substantially as small as an yttrium iron garnet while at the same time improving the temperature stability over that of the yttrium iron garnet. Moreover, a reduction in the saturation magnetization also occurs, thereby allowing some reduction in the size of the applied magnetic eld required to obtain the desired ferritte action.

In applications wherein temperature stability must be improved to an exceptional degree so that the magnetization can be held substantially constant over relatively wide temperature ranges, it may be desirable to select 4an yttrium-gadolinium garnet having a higher gadolinium content. For example, if the m01 percentages of gadolinium and yttrium are substantially equal (=0.5) the magnetization remains relatively constant over a fairly broad temperature range. Such temperature stability improvement is obtained at some expense inasmuch as the line width increases as the gadolinium content is increased. For many applications the sacrifice in line width is acceptable in view of the improved temperature characteristics. Moreover, the value of magnetization is further reduced, thereby allowing la further reduction in the applied magnetic fie-ld required.

Garnets that are fabricated in accordance with this range of values have weight percentage ranges as shown in Table II, wherein the gadolinium oxide weight percentage has a range from about percent to 45 percent, the yttrium oxide weight percentage has a range from about percent to about 40 percent and the ferrie oxide has a weight percentage from about 45 percent to 60 percent.

A third class of mixed crystals can be formed having the combined characteristics of the first and second classes described above. The formula for this third class can be expressed as (1A1 GdM)3Fe5O12 in which A is yttrium, lutecium or a combination of both and M is a rare earth element, such as samarium, having a large line width AH. The factors a and are independently varied over ranges from 0 to 1. `lt is readily seen therefore the line width AH of the mixed composition can be varied by varying a and the saturation magnetization Ms of the mixed composition can be independently varied by varying The preparation of the mixed crystals discussed above can be accomplished by using known ceramic techniques to obtain homogeneous materials with minimum porosity. A particular example is the formation of garnets from raw materials in the form of oxides. Mixtures of rare earth oxides (A203) and iron oxide ('Fe203) in the ratio of 3 mols to 5 mols, respectively, are milled with steel balls and water and after drying are pressed into a desired shape utilizing cer-amie or powder metallurgy techniques. Firing of the pressed material is done in air or in oxygen at tem eratures between 1200" C. and 1400 C. In some cases it may be desirable to perform presintering or calcination firing -at lower temperatures before the final sintering process. At approximately 1200 C. sintering just begins and at temperatures of 1350 C. to 1400 C. sintering takes place to give a bulk density up to 95 percent of the theoretical value. X-ray diffraction patterns indicate that mixed garnets are formed. High resistivities of the order of 105 to 1109 ohm-centimeter result. Other conventional methods such as extrusion may also be used in the formation of the mixed crystal garnets.

Other materials may be added to the garnet composition for such purposes as changing the sintering temperature by providing a material to tact as la ux or changing any of its other properties, such as resistivity. Such additions do not prevent or disrupt the indpendent variation of AH and -Ms and, hence, cannot be Iconstrued to limit the scope of this invention in any way.

Resonant measurements are made by using small samples of garnets mounted in a resonant cavity. The garnet absorption is measured as a function of applied D.C. magnetic field. Although the particular microwave characteristics of these materials may depend upon the `frequency at which the measurements are made, the general principle of having independently variable AH and Ms that is the essence of this invention remains unchanged.

This invention has important applications in microwave systems. The magnetization, Curie temperature and microwave properties at 10,000 megacycles are similar in the yttrium-samarium class of garnets to those in the manganese-magnesium -ferrites that are commonly used at this frequency. By appropriate combinations of yttrium and samarium the optimum compromise between band width and attenuation ratio is accomplished. In particular, very high attenuation ratios will be possible in narrow band width `systems by using lmaterials with small AH. At 3000-5000 megacycles it is Well known that the low saturation magnetization that is required to minimize losses in some devices is presently achieved only with materials having low Curie temperatures and/or `large line widths. Yttrium-gadolinium garnets give the required low saturation magnetization with small line rwidth and unchanged Curie temperature. At and below 11500 mc. isolators using the small line width materials are now -available with this invention and the lower limit of practical frequencies is correspondingly decreased. in addition, the use of such materials having low saturation magnetization offers the additional advantage of decreased losses at low field strengths.

The specific embodiment of the invention described here comprises combinations of yttrium and some rare earth metal oxides with iron oxide. It is possible to combine yttrium and rare earth metal oxides with oxides made up of iron and other elements which may in combination have suitable -ferromagnetic properties. For instance, the yttrium garnet may be formed according to the formula Y3(Fe1 aNa)5O12 where N may be, for example, aluminum or other element which in combination with iron has suitable ferromagnetic characteristics. Such a variation in the garnet structure in no Iway limits the scope of the invention because it is still possible to independently vary AH and Ms in such a combination,

In summation, although the specific embodiment of the invention as described herein comprises combinations of yttrium, gadolinium and samarium, it is undoubtedly true that many of the rare earth metals having atomic numbers 57 to 71 that can be formed into garnet crystal struetures can be used to bring about the desired AH and Ms characteristics. Measurements of line widths indicate that yttrium and lutecium garnets have small line widths, gadolinium garnet has a small line width except in the vicinity of room temperature at its magnetic compensation point, and the other rare earth metals capable of forming garnets have large line widths. Use of combinations of these elements according t0 the general formulas described herein, thus, allow the formation of garnets having independently variable AH and Ms.

The method of preparation of the invented materials, the use of additive materials to obtain characteristics independent of resonance and saturation magnetization phenomena, and the use of the invented compositions for any of the frequency ranges discussed above in no way limit the principle of the invention. Hence, the specific embodiment described in this specification is not to be construed as limiting the invention except as dened herein by the appended claims.

What is claimed is:

l. A composition consisting of about 5 to 45 percent by weight of gadolinium oxide, about 10 to 40 percent by weight of yttrium Oxide and about 45 to 60 percent by weight of ferrie oxide.

2. A composition consisting of about 20 percent by weight of yttrium oxide, about 32 percent by weight of gadolinium oxide and about 48 percent by weight of ferric oxide.

3. An yttrium-gadolinium garnet for use in a microwave device for providing a small line width consisting of about 7 percent by weight of gadolinium oxide, about 40 percent by weight of yttrium oxide, and about 53 percent by weight of ferrie oxide.

4. An yttrium-gadolinium garnet for use in a microwave device for providing a saturation magnetization that is stable over a temperature range from about room ternperature to about 200 C. consisting of about 40 percent by weight of gadolinium oxide, about l5 percent by weight of yttrium oxide, and about 45 percent by weight of ferrie oxide.

5. A composition of ferromagnetic material described by the formula (Y1 Sm)3Fe5O12 in which a is greater than zero and less than one and related to the ferromagnetic resonance line width AH by the equation AH :3,000a.

6. A composition of ferromagnetic material dened by the formula (Y1 Gd)3Fe5O12 in which ,8 is greater than zero and less than one and is substantially related to the saturation magnetization of said composition, 41rMs, by the relation 41rMs= 1900 (1-).

7. A composition of ferromagnetic material defined by the formula (Y1 Gd)3Fe5O12 in which is substantially related to the saturation magnetization of said composition, Ar11-MS, by the relation 41rMS=1900 (l-), and in which the value of is between .50 and .75.

8. A composition of ferromagnetic material described saturation magnetization of said material 41rMs by the by the formulae (Y1 GdSm) 3Fe5012 in which a is relation 41rMs=l900 (l-). greater than zero and less than one and is substantially related to the ferromagnetic resonance line width AH References cned m the me of dus patent by the relation AH=3,000a and in which is greater than 5 UNITED STATES PATENTS zero and less than one and is substantially related to the 3,003,966 Van Uitert Oct. 10, 1961 

5. A COMPOSITION OF FERROMAGNETIC MATERIAL DESCRIBED BY THE FORMULA (Y1-ASMA)3FE5O12 IN WHICH A IS GREATER THAN ZERO AND LESS THAN ONE AND RELATED TO THE FERROMAGENTIC RESONANCE LINE WIDTH $H BY THE EQUATION $H=3,000A.
 6. A COMPOSITION OF FERROMAGNETIC MATERIAL DEFINED BY THE FORMULA (Y1-BGDB)FE5O12 IN WHICH B IS GREATER THAN ZERO AND LESS THAN ONE AND IS SUBSTANTIALLY RELATED TO THE SATURATION MAGNETIZATION OF SAID COMPOSITION, R$M5, BY THE RELATION 4$MS=1900(1-B),
 8. A COMPOSITION OF FERROMAGNETIC MATERIAL DESCRIBED BY THE FORMULAE (Y1-A-BGDBSMA)3FE5O12 IN WHICH A IS GREATER THAN ZERO AND LESS THAN ONE AND IS SUBSTANTIALLY RELATED TO THE FERROMAGNETIC RESONANCE LINE WIDTH $H BY THE RELATION $H=3,000A AND IN WHICH B IS GREATER THAN ZERO AND LESS THAN ONE AND IS SUBSTANTIALLY REALTED TO THE SATURATION MAGNETIZATION OF SAID MATERIAL 4$MS BY THE RELATION 4$MS=1900(1-B). 